Use of lasers in industrial manufacturing environments is becoming increasingly widespread and such use includes welding of automotive body panels. Such body panels, however, are increasingly being produced from lightweight sheet metals such as aluminum and magnesium, which are traditionally not conducive to high production laser welding.
In general, laser welding is a joining process wherein coalescence of substrate materials is produced by heating the substrate materials to suitable temperatures without the application of pressure, and with or without the use of a filler material. More specifically, in a typical laser welding process, steel members are assembled with facing surfaces in juxtaposition, for example, to form a lap joint, wherein an outer surface of one of the steel members is irradiated with a laser beam to melt and fuse the steel members at the facing surfaces. In contrast to other welding processes, such as resistance welding, that generate heat concentrated at the facing surfaces, laser welding heats a zone extending from the irradiated outer surface down below the facing surfaces to create a pool of molten metal within both members that, upon solidification thereof, forms a weld nugget or bead that joins the two sheet members together.
Additionally, some laser welding applications require a technique known as “keyholing” that involves use of relatively high power lasers to make relatively deep penetrations at increased welding speeds. Keyholing involves heating the zone of laser focus above the boiling point of the substrate materials to form a vaporized hole in the substrate materials. The vaporized hole becomes filled with ionized metallic gas and becomes an effective absorber, trapping most of the energy from the laser into a cylindrical volume, known as a keyhole. Instead of heat being conducted mainly downward from the outer surface of one of the substrate materials, it is conducted radially outward from the keyhole, forming a molten region surrounding the ionized metallic gas. As the laser beam moves along the substrate materials, the molten metal fills in behind the keyhole and solidifies to form a weld bead.
While laser welding is widely successful in joining steel substrates, it has met with limited success in joining aluminum or magnesium substrates. Laser welding involves light beams and, thus, laser welding suffers from problems with reflective material such as aluminum. Additionally, aluminum presents several metallurgical difficulties because some common alloying elements therein, like zinc and magnesium, have very high vapor pressures and, thus, tend to boil out of a molten weld trough under typical laser welding conditions. Besides depleting the alloy content of the weld, this “boil out” condition leads to keyhole instability and high levels of porosity in laser welds, particularly where the depth of the keyhole is greater than the width of the weld bead. Also, lap-welded joints have a particular problem with out-gassing of coatings or contaminants on the aluminum substrates that leads to weld bead porosity.
In order to minimize porosity in the resultant weld nugget or bead it has heretofore been common practice to add filler metal to laser welding processes, or to pulse the laser in an attempt to alter the solidification rate of the molten weld trough. Regardless of the particular type of welding process used, most aluminum alloys must be welded with a filler metal having a different composition than the substrate aluminum to avoid weld cracking and porosity. Filler metal, unfortunately, is difficult to use with lasers because it is very difficult to get filler metal wire into the tiny melt zones that most lasers produce. Furthermore, attempts to optimize the welding heat input by pulsing the laser, and thereby controlling the rate of solidification of the molten weld trough, have not met with good results. In theory, it should be possible to pulse the laser on and off at a predetermined rate in order to permit gradual or slower solidification of the molten weld trough and thereby avoid porosity. In practice, however, this process of intrinsically regulating the temperature of the molten weld trough by pulsing the laser simply does not solve the problem of porosity.
Thus, there remains a need for a method of laser welding aluminum or magnesium substrates that does not require use of filler metal, yet results in substantially porosity-free welds.